drm/mgag200: Rename cursor functions to use mgag200_ prefix

Although the driver source code is fairly inconsistent wrt naming, the
prefix should be mgag200. Rename cursor functions accordingly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927091301.10574-2-tzimmermann@suse.de